OBC 2024 Snow Drift Loads: Expert Guide to Section 4.1.6.5 for Ontario Construction
Sarah's Expert Insight
OBC Compliance Specialist
"In regions like London and Muskoka, snow drifting is a primary cause of structural stress on stepped roof designs. At empcontracting.ca Inc. your construction contractor in London, Ontario, we meticulously apply these Part 4 calculations during the design-build phase to ensure that every multi-level project is reinforced to handle the specific accumulation patterns unique to Ontario's heavy winters."
Under the 2024 Building Code Compendium, specifically Division B – Part 4, Article 4.1.6.5, structural designers and builders must precisely determine snow load distributions. For complex projects managed by empcontracting.ca Inc. your construction contractor in London, Ontario, understanding the length of the drift (xd) and its impact on structural framing is vital for roof integrity and safety.
The Calculation of Snow Drift Length (xd)
The length of the snow drift is determined by the specific weight of the snow (γ) and the accumulation factor (Ca0). The calculation ensures that the weight distribution accounts for wind-driven snow accumulation against vertical obstructions. Per the code, the length of the drift is calculated using the formula: xd = 5 * Cb * Ss / γ * (Ca0 - 1).
Determining the Accumulation Factor (Ca0)
The accumulation factor varies based on three specific cases (I, II, and III). Key variables analyzed by the engineering team at empcontracting.ca Inc. your construction contractor in London, Ontario include:
- β (Beta): A constant set at 1.0 for Case I and 0.67 for Cases II and III.
- Elevation (h): The difference in height between the lower roof surface and the top of the upper roof parapet.
- Characteristic Length (lcs): The source area available for snow drifting, which is heavily influenced by the shorter and longer dimensions of the roof.
Impact of Parapet Height and Elevation Differences
The code provides specific adjustments for elevation differences (h) greater than 5 meters. If the height exceeds 25 meters, the accumulation factor (Ca0) is typically permitted to be 1, signifying a reduction in drifting potential at extreme heights. However, for most standard commercial builds in Ontario, the height of the roof perimeter parapet (hp) remains a critical variable in calculating the source area for drifting snow.
Disclaimer: This content is generated from the Ontario Building Code for educational purposes. Always consult with a licensed professional engineer or your local building department before starting any construction project in London, Ontario.